Hydrophobic nanostructured thin films

ABSTRACT

Provided herein are the polymers shown below. The value n is a positive integer. R 1  is an organic group, and each R 2  is H or a chemisorbed group, with at least one R 2  being a chemisorbed group. The polymer may be a nanostructured film. Also provided herein is a method of: converting a di-p-xylylene paracyclophane dimer to a reactive vapor of monomers; depositing the reactive vapor onto a substrate held at an angle relative to the vapor flux to form nanostructured poly(p-xylylene) film; reacting the film with an agent to form hydrogen atoms that are reactive with a precursor of a chemisorbed group, if the film does not contain the hydrogen atoms; and reacting the hydrogen atoms with the precursor. Also provided herein is a device having a nanostructured poly(p-xylylene) film on a pivotable substrate. The film has directional hydrophobic or oleophobic properties and directional adhesive properties.

This application is a divisional application of U.S. patent applicationSer. No. 14/018,507, file on Sep. 5, 2103, which is a divisionalapplication of U.S. Pat. No. 8,535,805, issued on Sep. 17, 2013, whichclaims the benefit of U.S. Provisional Patent Application No.61/048,475, filed on Apr. 28, 2008. The provisional application, allpublications referenced therein, and all other publications and patentdocuments referenced throughout this nonprovisional application areincorporated herein by reference.

TECHNICAL FIELD

The disclosure is generally related to hydrophobic nanostructured thinfilms.

DESCRIPTION OF RELATED ART

Hydrophobicity and adhesive properties of nanostructured surfaces areimportant for many practical applications, such as the handling of smallliquid droplets, inducing selective permeability in a membrane, and theoperation of wall-climbing robots. The methods of preparingsuperhydrophobic and adhesive surfaces include chemical etching andtemplate-based techniques. However, the simultaneous control of bothwettability and adhesion properties of superhydrophobic surfaces has notbeen studied in detail.

[2.2]Paracyclophane was first prepared in 1949 by Brown and Farthing(Brown et al., Nature 1949, 164, 915-916) and systematicallyinvestigated by Cram and co-workers from 1951 onward (Cram et al., J.Am. Chem. Soc. 1951, 73, 5691-5704). Chemically, [2.2]paracyclophane isa dimmer of two p-xylylene groups (a layered π system) that have anunusual 3D aromatic structure compared to that of the planar benzenering. Applications of [2.2]paracyclophane include molecular machinesrealized through supramolecular assembly (Anelli et al., Angew. Chem.,Int. Ed. Engl. 1991, 30, 1036-1039) and polymeric thin films(poly(p-xylylene)) of cyclophanes for biomedical purposes (Gorham, J.Polym. Sci., Part A: Polym. Chem. 1966, 4, 3027-3036).

Only a few examples of nanostructured PPX films have been described inthe literature, including PPX polymer brushes (Lahann et al., Macromol.Rapid Commun. 2001, 22, 968-971) and template PPX fibers (Bognitzki etal., Adv. Mater. 2000, 12, 637-640).

Nanoporous films have generated great interest due to their unusualphysical and chemical properties arising from their high surface areaand their nanoscale dimension. One important property is controlling thehydrophobicity of surfaces for antifouling and biomedical applications.Examples of nanostructured materials for tuning the hydrophobicity havebeen shown for self cleaning, anti-sticking and anti-contamination. Themethods of preparing and tuning hydrophobic surfaces are template basedtechniques, plasma treatment, chemical deposition, layer-by-layerdeposition and colloidal assembly. However, all these methods are eithercostly or difficult to prepare at an industrial scale. Halogenderivatives of PPX are hydrophobic and used in variety of applications,such as in the automotive, medical, electronics, and semiconductorindustries. They are known for their gas phase chemical vapor depositionand conformal coating (pinhole-free) at room temperature.

Superhydrophobic surfaces (water contact angle θ_(w)>150°) have beenwidely studied to develop materials with unique properties such asself-cleaning and/or antifouling behavior (Feng et al., Adv. Mater.2006, 18, 3063-3078; Sun et al., Acc. Chem. Res. 2005, 38, 644-652).Normally, superhydrophobic surfaces have higher contact angles and verylow water droplet roll-off angles (<5°). A new class of superhydrophobicbut adhesive surfaces has been recently reported (Guo et al., Appl.Phys. Lett. 2007, 90, article no. 223111; Hong et al., J. Am. Chem. Soc.2007, 129, 1478-1479; Jin et al., Adv. Mater. 2005, 17, 1977-1981).These surfaces are prepared either by surface etching or by microscopicstructures of hydrophobic materials. Although these nanostructuredsurfaces have high water contact angles (θ_(w)>150°), a water dropadheres to a surface even if the surface is tilted upside down (i.e.,180°). This strong adhesion has been attributed to van der Waals and/orthe capillary force interactions between the nanostructured film surfaceand water. Similarly, biological structures built using molecularprotein machinery in nature also show superhydrophobic and adhesivesurface features. For example, the attractive forces that hold geckos tosurfaces are van der Waals interactions between the finely dividedkeratinous fibers (˜5×10⁵ on each foot) and the surfaces themselves(Huber et al., Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 16293-16296;Autumn et al., Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12252-12256;Ruibal et al., J. Morphol. 1965, 117, 271-293).

BRIEF SUMMARY

Disclosed herein is a polymer comprising the repeat unit shown informula (1), wherein n is a positive integer. The polymer is ananostructured film made by: converting a trifluoroacetyl-di-p-xylyleneparacyclophane dimer to a reactive vapor of monomers, the reactive vaporhaving a flux, and depositing the reactive vapor under vacuum onto asubstrate held fixed in a specific angle of orientation relative to thevapor flux to form nanostructured poly(trifluoroacetyl-p-xylylene)(PPX-COCF₃) film.

Also disclosed herein is a polymer comprising the repeat unit shown informula (2), designated PPX-COHCF₃, wherein n is a positive integer.

Also disclosed herein is a polymer comprising the repeat unit shown informula (3), wherein n is a positive integer, R¹ is an organic group,and each R² is H or a chemisorbed group. The polymer comprises at leastone chemisorbed group. All these polymers may also be represented by thegeneral structure shown in formula (4), where R_(a) and R_(b) arehydrogen or an organic group. A suitable range of values for n is 5 to10,000.

Also disclosed herein is a method comprising: converting a di-p-xylyleneparacyclophane dimer to a reactive vapor of monomers, the reactive vaporhaving a flux; depositing the reactive vapor under vacuum onto asubstrate held fixed in a specific angle of orientation relative to thevapor flux to form nanostructured poly(p-xylylene) film; reacting thenanostructured poly(p-xylylene) film with an agent to form hydrogenatoms attached to the poly(p-xylylene) film that are reactive with aprecursor of a chemisorbed group, if the deposited film does not containthe hydrogen atoms; and reacting the hydrogen atoms with the precursorof the chemisorbed group.

Also disclosed herein is a device comprising a nanostructuredpoly(p-xylylene) film on a pivotable substrate. The film has directionalhydrophobic or oleophobic properties and directional adhesiveproperties.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention will be readily obtainedby reference to the following Description of the Example Embodiments andthe accompanying drawings.

FIGS. 1A-C shows surface AFM scans for structuredpoly(chloro-p-xylylene) (PPX-Cl) films. Surface roughnesses are FIG. 1A:8.1 nm, FIG. 1B: 37.6 nm and FIG. 1C: 77.5 nm. (Scale bars for the AFMscans: X:1 μm/div, Y:1 μm/div, Z:100 nm/div).

FIG. 2A shows a cross-sectional SEM image of the structured PPX-COCF₃film. Shapes of water droplets on the structured PPX-COCF₃ film withdifferent tilt angles: FIG. 2B: 0°, FIG. 2C: 90°, and FIG. 2D 180°.

FIG. 3A shows FTIR/ATR spectra of the structured PPX-COCF₃ film areshown for three conditions: (1) before lithium aluminum hydride (LAH)reduction, (2) after LAH reduction, and (3) after 3 h offluoroalkyltrichlorosilane chemisorption. FIG. 3B shows that waterdroplets adhere well to the surface for conditions 1 and 2 but becomenonadherent (roll-off angle<5°) for condition 3. The contact angles forconditions 1-3 are 136°, 78°, and 152°, respectively. (2) Thedisappearance of the hydroxyl IR band at 3500 cm⁻¹, as shown in spectrum3 of FIG. 3A.

FIG. 4A shows a cross-section SEM image and a top-surface AFM image of ananostructured PPX-Cl thin film fabricated by deposition for 10 min on asilicon substrate. FIG. 2B shows a top-surface AFM image of the samefilm showing nanowires that are approximately 50-100 nm in diameter(scale shows the height). FIG. 4C shows a transmission IR spectra of ananostructured PPX-Cl thin film and a planar PPX-Cl thin film. FIG. 4Dshows an optical image of a water drop on the top surface of ananostructured PPX-Cl thin film, the apparent contact angle being 100°.FIG. 4E shows measured and calculated diffraction patterns of ananostructured PPX-Cl thin film. FIG. 4F shows crystal structure ofPPX-Cl, wherein hydrogen atoms are not shown for clarity and the firstand last carbon atoms in the crystal lattice represent the repeat unitalong the polymer chain (carbon and chlorine shown in gray and blackrespectively).

FIG. 5 shows X-ray photoelectron spectroscopy (XPS) analysis ofheptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane chemisorbed onnanostructured PPX-COHCF₃ film. Si(2p) spectrum shows the chemisorptionof the silane on the film. XPS data were acquired using an Axis UltraXPS system (Kratos) with a monochromatic Al Kα X-ray source, 20 eV passenergy (700 μm×300 μm hybrid sample spot size), and 90° take-off angleunder high vacuum conditions (10⁻⁹ Torr).

FIG. 6 shows an embodiment of a device for moving drops of a liquid.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description, for purposes of explanation and notlimitation, specific details are set forth in order to provide athorough understanding of the present disclosure. However, it will beapparent to one skilled in the art that the present subject matter maybe practiced in other embodiments that depart from these specificdetails. In other instances, detailed descriptions of well-known methodsand devices are omitted so as to not obscure the present disclosure withunnecessary detail.

The present disclosure relates generally to materials and methods forcontrolling adhesion of nanostructured thin films. The presentdisclosure further relates to tuning the adhesion of hydrophobicnanostructured poly(p-xylylene) (“PPX”) surfaces. The present disclosurefurther relates to a method of depositing a nanostructured PPX thin filmexhibiting tunable, controlled liquid wettability and adhesionproperties via the oblique angle vapor deposition and subsequentpolymerization of a paracyclophane precursor onto a substrate surface. Amethod and system for forming and using the PPX films is described andset forth in U.S. Patent Application Publication No. 2007/0148206.

The hydrophobic surface properties and controlled topography ofstructured PPX films, as measured by water wettability, are determinedby various factors, including surface chemistry, film composition, andsurface roughness. Disclosed herein is the fabrication of veryhydrophobic surfaces and control over adhesion properties via nanoscalemodulation of roughness, changes in composition, and alteration of thesurface chemistry of PPX films. The formation of superhydrophobicsurfaces through the chemisorption of fluoroalkylsiloxane coatings tohydroxyl sites created on the nanostructured PPX surface is alsoillustrated. The ability to control both hydrophobicity and adhesionusing nanostructured PPX films may lead to a new generation of coatingswith applicability ranging from self-cleaning surfaces to robotics.

Films may be made by a bottom-up process based on oblique angledeposition as a simple, robust method for controlling the morphology ofPPX films (Cetinkaya et al., Polymer 2007, 48, 4130-4134; Demirel, M.C.; Boduroglu et al., A. Langmuir 2007, 23, 5861-5863; Demirel et al.,J. Biomed. Mater. Res. Part B 2007, 81, 219-223; U.S. Patent ApplicationPublication Nos. 2007/0148206 and 2008/0268226). In this process,monomer vapors produced by the pyrolysis of chemically functionalized[2.2]paracyclophane precursors are directed at an oblique angle toward asurface to initiate PPX growth. Inclined deposition induces theisotropic growth of PPX nanofibers as a result of high surface diffusionand geometric shadowing effects, leading to a nanostructured PPX filmcomprising clusters of ˜50-200 nm diameter nanocolumns. This approachallows tuning of the chemical properties of the nanostructured PPXsurface (via the judicious choice of the functionalized paracyclophaneprecursor) and film morphology (via control of the film depositionconditions) to control the physicochemical properties of the resultingPPX films, such as hydrophobicity, porosity, and crystallinity.

Disclosed herein is the control of adhesive properties of severalhydrophobic PPX films deposited by the oblique angle method. Alsodisclosed are multiple ways of tuning the hydrophobicity of thestructured PPX surfaces. This method allows a change not only in thesurface roughness but also in the surface chemistry of the material.Also disclosed is the fabrication of superhydrophobic films through thechemisorption of fluoroalkylsiloxane coatings to hydroxyl sites createdon the PPX film surface and the control of both water wettability andadhesion through appropriate choices of PPX surface chemistry androughness. The ability to simultaneously control surface chemistry andfilm morphology shown here makes these nanostructured PPX filmspotential candidates for use in antifouling and biomedical applications.

Thin films of poly(chloro-p-xylylene) (PPX-Cl) grown by the pyrolysisand evaporation of dichloro-[2.2]paracyclophane in an evacuated chambercontain free-standing, slanted, parallel columns that are 50 μm long andare assemblies of 50- to 100- to 150-nm-diameter nanowires, which thuscan have an aspect ratio as high as 1000:1. The nanostructured thinfilms organize spatially with a chemical structure similar to that ofplanar PPX-Cl thin films, but the former also possess nanostructuredmorphology. Nanostructured thin films of PPX and its derivatives may beuseful as functionalized interfaces for antifouling coatings andbiomedical devices. The production technique does not require any mask,lithography method, or surfactant for deposition.

The nanostructured PPX thin films may have two advantages over planar(flat) PPX thin films. First, the nanostructure enhances the surfacearea, thereby increasing the efficiency of functionalization. Second,novel chemical properties can be obtained by the co-deposition of two ormore types of PPX monomers with different side groups, which can beesters, ketones, amines, lactones, and so forth.

PPX film chemistry is a parameter for tuning the hydrophobicity ofnanostructured films. Three PPX nanostructured columnar thin films,poly(bromo-p-xylylene) (PPX-Br) andpoly(o-trifluoroacetyl-p-xylylene-co-p-xylylene) (PPX-COCF₃) andpoly(chloro-p-xylene) (PPX-Cl), were deposited obliquely on glasssubstrates at an angle α (=10°). Microscopic features of films fromthree different substrates appear to have close similarity.

Superhydrophic surfaces of PPX may be created by changing the surfacechemistry or surface roughness. The hydrophobicity of the PPX film mayincrease as the film become more electronegative and the surfaceroughness increases. The nanostructured PPX columnar thin films may beutilized in antifouling and biomedical applications by controllingsurface topology, chemistry and film morphology at the same time.

PPX thin films have a wide range of usage such as chemical and corrosionresistant coatings, capacitor dielectrics, moisture barriers, electricalinsulators and dry lubricants, anti-friction layers in MEMS.Nanostructured PPX coatings may improve surface properties oftraditionally coated medical devices by providing superhydrophobic andself-decontaminated surfaces. Potential advantages of nanostructured PPXas biomedical coatings include:

-   -   (i) Their chemistry and porosity are controllable, and so it is        possible to engineer not only the surface properties of STFs but        also 3-dimensional scaffolds;    -   (ii) They can be made out of many FDA-approved polymeric        materials and can be endowed with transverse architectures to        provide the best possible substrate and coating material for        biomedical devices.

As used herein, the following terms have the meanings shown.

Nanostructured—having structures of dimensional size between molecularand microscopic PPX or parylene—poly-(p-xylylene)

Organic—containing at least carbon and hydrogen atoms, and optionallyoxygen atoms

Chemisorbed—adsorption involving a chemical (e.g. covalent) linkagebetween adsorbent and adsorbate. The chemisorption may be limited to thesurfaces of a film, including a nanostructured film. The chemisorptionmay also partially or fully penetrate the film.

Functionalized—altered on the chemical level, in particular by addingone or more chemical substituents to a polymer (for example a PPXpolymer) backbone, for example, ester, amine, ketone, lactone, halogen,hydroxyl, acetyl, etc., where

Silane—Si(R)4-n(X)_(n) wherein R is hydrogen or unsubstituted orsubstituted alkyl or aryl and X is halogen or alkoxide and n is apositive integer from 1-3.

Rough Surface—one specific possible type of nanostructured thin filmtopographic morphology

Nanocolumns—approximately parallel aligned straight wire structures ofPPX having individual column wire diameters less than about 250 nm. Theyare formed by the oblique angle deposition method whereby a flux ofpyrolyzed para-[2.2]cyclophanes precursor vapor impinges on anon-rotating planar substrate surface to be coated with thenanostructured PPX film at an angle α with respect to the substratesurface plane, where 0°<α<90° and a is most preferably 10°<α<30°.

Nanohelices—approximately parallel aligned helical wire structures ofPPX having individual wire diameters of less than about 250 nm. They areformed as described for nanocolumns with deposition occurring on aplanar substrate rotating about an axis defined by the normal to thesubstrate surface at a constant angular velocity such that the vaporflux impingement angle α is unchanged during the deposition process.

Nanochevrons—approximately parallel aligned zigzag wire structures ofPPX having individual wire diameters of about 250 nm or less. They areformed as described for nanocolumns by initial vapor flux deposition atangle α for a fixed time, t. At time=t the planar substrate is rotated180° about the normal to its surface and fixed in its new position.Vapor flux deposition is continued, at angle α with respect to thesubstrate surface plane, for an additional time interval, t. The cycleof deposition for fixed time, t, followed by 180° rotation is continueduntil a film of desired thickness is formed.

Channels—interstitial regions between individual spaced nanostructureson a nanostructured thin film surface

nm—nanometer

Contact angle—the angle formed between a water droplet and a solid statesurface. Measurements here typically refer to the sessile water dropcontact angle, which is the contact angle a water drop quietly resting asubstrate held parallel to the ground makes with the surface of thatsubstrate.

Roll-off angle—This is the minimum angle to which a planar substratebearing a water droplet on its surface and held parallel to the groundmust be tilted for the water droplet to roll off or slide off thesubstrate surface. This angle is also sometimes called the slidingangle.

LAH—lithium aluminum hydride or LiAlH₄

THF—tetrahydrofuran

(Organo)siloxane or siloxane—Si(R)_(4-n)(O)_(n) wherein n is an integerfrom 1-3 and R is an optionally substituted alkyl or aryl group or H

As used herein, a hydrophobic substituent group is a group such that,when the paracyclophane vapor is deposited as a flat non-nanostructuredthin film, yields a parylene flat surface having a sessile water dropcontact angle greater than ˜70 degrees. A hydrophilic substituent willtherefore have a contact angle less than ˜70 degrees for water.

The nanostructured PPX thin films disclosed herein are deposited on astationary substrate where polymerization occurs on the substrate, froma directional vapor source in an evacuated chamber. [2.2]Paracyclophanederivatives are first converted to a reactive vapor of monomers ofp-xylylene by pyrolysis. The substrate is oriented obliquely relative tothe vapor flux, typically at an angle α˜10° in FIG. 5, which creates aporous, low-density thin film of columns inclined at an angle of φ>α.Without limiting the present subject matter to any particular theory,the growth of nanoporous columnar thin films may be governed by threemechanisms: (i) geometrical self-shadowing, (ii) surface diffusion alongthe substrate of incoming adatoms constituting the vapor, and (iii) bulkdiffusion leading to oriented crystallization (Lakhtakia et al.,Sculptured Thin Films: Nanoengineered Morphology and Optics; SPIE Press:Bellingham, Wash., 2005). Such thin films of metals, semiconductors, anda few organic dielectrics have been deposited by oblique-angle vapordeposition methods (Lakhtakia, Id.; Tsoi et al., Langmuir 2004, 20,10771-10774). Similar thin films of polymers have been grown by anoblique-angle molecular beam deposition method (Cai et al., Adv. Mater.1999, 11, 745-749) on the atomistic length scale but not on largerlength scales. The formation of the nanostructured PPX thin films mayhave been influenced by a combination of nucleation (common in thinfilms (Petrov et al., J. Vac. Sci. Technol., A 2003, 21, S117-S128;Demirel et al., Phys. Rev. Lett. 2003, 90, 016106)) with bond formation(i.e., polymerization), in addition to the aforementioned threemechanisms.

Wettability measurements using various solvents have long been used as ameans to estimate a material's surface energy and understand surfaceenergy variations with changes in the material's chemistry, composition,and morphology. In particular, the contact angle formed by a sessilewater droplet (θ_(w)) resting on a substrate surface provides aconvenient means to assess the relative hydrophobicity of that surface.The value of θ_(w) is known to depend on both the chemical compositionand the roughness of the surface, as described previously by Cassie andWenzel, respectively (Wenzel, Ind. Eng. Chem. 1936, 28, 988-994; Cassieet al., Trans. Faraday Soc. 1944, 40, 0546-0550). Nanostructured PPXfilms, as described here, offer the opportunity to vary both surfacechemistry and roughness simultaneously and precisely as a means tocontrol the hydrophobicity and adhesive properties of such films. Threeparameters can influence the hydrophobicity and adhesion of PPX films:(i) film roughness, (ii) chemical composition of the film, and (iii)direct chemical modification of the film surface.

On a nanostructured surface, there are three wetting states. As usedherein “wettability” refers to surface wetting by water or other liquidssuch as oils. A water drop can fully penetrate into the nanostructures,which is known as the Wenzel state, or the liquid can be suspended onthe nanostructures, which is known as the Cassie-Baxter state. The thirdstate is an intermediate state between the Wenzel and Cassie modes. Whenthe wetting behavior changes from the Cassie mode to the Wenzel mode,the liquid droplet will at least partially fill the grooves of the roughsubstrate with a decrease in the apparent contact angle. Consequently,adhesive forces between the surface and water droplet play a key role inthe third state.

To better understand the range of conditions that characterize PPXwetting behavior in these three states, the effects of film roughness onθ_(w) using poly(chloro-p-xylylene) (PPX-Cl) films of variousthicknesses was investigated. θ_(w) (97.9°, 103.9°, and 111.2°) wasmeasured as a function of surface roughness (8.1, 37.6, and 77.5 nm,respectively). The increase in surface roughness as the film becomesthicker is inherent to oblique angle deposition techniques. The surfacehydrophobicity increases (i.e., water wettability decreases) as afunction of surface roughness as expected, but the data do not obey theWenzel or Cassie-Baxter equation. For example, Wenzel behavior isdescribed by Eq. (1), where r is the measured surface roughness factorand θ_(w) ^(p) is the contact angle on a planar PPX-Cl surface (i.e.,87°, see Table 1):

cos θ_(w)=r cos θ_(w) ^(p)   Eq. (1)

Eq. (1) predicts θ_(w) values of 86.5, 84.7, and 82.5° as the roughnessincreases. Similarly, Cassie-Baxter behavior is governed by Eq. (2),where f is the fraction of the water droplet in direct contact with thesubstrate (i.e., not suspended over air) on the rough surface:

cos θ_(w) =f(1+cos θ_(w) ^(p))   Eq. (2)

AFM mapping of the tops of the PPX-Cl fibers is shown in FIGS. 1A-C,which provide f values of 0.63, 0.67, and 0.68 for the same filmscorresponding to predicted θ_(w) values of 109.5, 107.4, and 106.3° fromEq. (2), respectively, that again differ markedly from measured θ_(w)values. These results suggest that van der Waals and/or capillary forcessufficient to induce wetting behavior associated with the intermediatestate are present for the structured PPX-Cl films.

Changes in the chemical composition of the PPX film associated with theuse of [2.2]paracyclophanes bearing other hydrophobic substituents, suchas the trifluoracetyl group, also produce nanostructured films capableof exhibiting this intermediate wetting state. For example, FIG. 2Ashows a cross-sectional scanning electron microscope (SEM) image of astructured poly(trifluoroacetyl-p-xylylene) (PPX-COCF₃) film depositedusing the trifluoroacetyl-di-p-xylylene precursor on a siliconsubstrate. Very clearly, the SEM images confirm that the structuredPPX-COCF₃ film consists of assemblies of parallel columns. Theinclination angle of the columns is 61°. Although the structuredPPX-COCF₃ film morphology, as indicated by the diameter of its pillars,is similar to that of the PPX-Cl film (Table 1), it is more hydrophobic(θ_(w)=135.7°; Table 1 and FIG. 2B) than the PPX-Cl film (θ_(w)=119.3°,Table 1). However, good adhesion of the water droplet placed on thesurface is observed for both films. For example, even when the filmswere tilted vertically or flipped upside down, as shown in FIGS. 2C, D,respectively, for the PPX-COCF₃ film, the water droplet did not slidefrom the surface. However, the planar PPX-COCF₃ film has a staticcontact angle of 88° (Table 1), and the roll-off angle is 25°. Thestructured PPX-COCF₃ film is composed of approximately ˜10⁷ alignedcolumns (approximately 120-170 nm in diameter) per square millimeter.Clearly, both increased surface hydrophobicity and adhesion are due tonanostructure.

TABLE 1 Static Contact Angle Measurements for PPX Films average columnSubstrate Type Planar (deg) Structured (deg) size (nm) PPX-COCF₃ 88 ± 3135.7 ± 4.0 160 ± 7  PPX-Cl 87 ± 2 119.3 ± 1.2 140 ± 11 PPX-Br 80 ± 2115.2 ± 1.4 188 ± 10

Similar adhesive behavior is also noted for structuredpoly-(bromo-p-xylylene) (PPX-Br) films prepared by the obliquedeposition of the dibromo-di-p-xylylene precursor at the α=10° incidentangle geometry used for the PPX-Cl and PPX-COCF₃ films, even though the(˜188±10)-nm-diameter PPX-Br columns are somewhat larger than those ofthe other films. Table 1 summarizes the wettability behavior andstructural features (i.e., average column diameters) observed for eachfilm.

Clearly, high surface adhesion is observed even for PPX films havingdiverse chemical compositions. The hydrophobic natures of the planar aswell as structured forms of the PPX-derivative films are exemplified bythe contact-angle goniometry results, which range from 80° to ˜136°depending on the type of film. In general, every planar film is lesshydrophobic than its structured counterparts as a result of theroughness differences; the structured PPX films are very rough comparedto the planar PPX films. This is expected because planar films form aconformal surface, and their surface roughness is very small.Additionally, structured PPX-COCF₃ exhibits superior hydrophobicitycompared to the remaining five films as a result of the CF₃ group in thepolymer and the surface roughness associated with the film'snanostructure. It is noteworthy that all three structured PPX surfacesexhibit good adhesive interactions with water droplets. Such behaviorsuggests that there exists a range of surface energies, as measured byθ_(w) (˜115-136°) and determined by the PPX surface chemical compositionand morphology (Table 1), for the fabrication of hydrophobic, adhesivematerials of these types.

Direct chemical modification of the native PPX surface represents anadditional means to control the hydrophobicity and adhesive propertiesusing the nanostructured PPX films. For example, it is generally knownthat the chemisorption of fluoroalkyltrichlorosilanes to hydroxyl groupspresent on appropriately roughened substrate surfaces, such as ZnO,provides a convenient means to fabricate superhydrophobic surfaces (Liet al., Nanotechnology 2006, 17, 238-243; Li et al., J. ColloidInterface Sci. 2005, 287, 634-639). Superhydrophobic surfaces satisfytwo important criteria. They exhibit (1) very high θ_(w) values (>150°)and (2) very low water droplet roll-off angles (<5°).

The surface of the nanostructured PPX-COCF₃ film may be reacted withlithium aluminum hydride (LAH) (or other suitable reducing agent) in THFto reduce the carbonyl group to a secondary alcohol. After thereduction, θ_(w) of the film, referred to hereafter as PPX-COHCF_(3,)was measured to be 78°, which is consistent with the replacement of theketone by a more hydrophilic hydroxyl group. The IR spectra of the filmsshown in FIG. 3A confirmed the reduction of the ketone to the alcohol.The ketone peak of the untreated PPX-COCF₃ film observed at 1712 cm⁻¹(spectrum 1) disappeared, and an OH band at 3500 cm⁻¹ (spectrum 2)ascribed to the secondary hydroxyl group appeared after the LAHtreatment. The chemistry of the surface layer(s) of the nanostructuredPPX-COCF₃ film of formula (1) is thereby altered to that of a PPX-COHCF₃species of formula (2) after the LAH treatment.

A chemically reactive group, designated R², can be reacted with the OHgroup of the PPX-COHCF₃ formed on the outer layer(s) of thenanostructured PPX-COCF₃ film to chemisorb said R² group to said OHsites of the PPX-COHCF_(3.) Such a modified PPX-COHCF₃ has the structureshown in formula (5). Formulas (6)-(10) show various chemisorbed groupsmade by reacting the PPX-COHCF₃ with a precursor of a chemisorbed group.

Formula (6) shows the product of reaction with afluoroalkyltrichlorosilane, where R³ is a fluorinated alkyl group and mis a positive integer. The R³SiCl₃ groups hydrolyze due to trace waterin the air and on the PPX surface (water associated with the OH groupsmade from the LAH reaction) to form R³Si(OH)₃ groups plus HCl gas. TheseOH groups then condense with themselves or the PPX surface OH groups viaelimination of water to form the Si—O—C and Si—O—Si bonds. Thus, atleast some, though not necessarily all of the silicon atoms areincorporated in to a polysiloxane. Suitable fluoroalkyltrichlorosilanesinclude, but are not limited to, those having a C₆-C₁₅ alkyl groupsubstituted with from 0 to 17 fluorine atoms andheptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane. The hydroxylsites created by the reduction of surface ketone groups of thestructured PPX-COCF₃ films by LAH react readily with hexane solutions ofheptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane to conformallychemisorb this organotrichlorosilane onto the polymer surface.

Formula (7) shows the product of reaction with trimethoxyallylsilane,which is similar to the above reaction. Formula (8) shows thecopolymerization product of formula (7) with N-isopropylacrylamide. Thestructure represents a random copolymer with m and p being positiveintegers.

Formula (9) shows the product of reaction with a polyisocyanate. R⁴ isan organic group. Formula (10) shows the subsequent reaction with apolyethylene glycol (PEG), where p is a positive integer. While thisformula shows linear PEG, a branched PEG may also be used. A suitablediisocyanate is hexamethylene diisocyanate (HMDI).

Other suitable R¹ groups can contain an a carboxyl group or an aminegroup. In general, all these polymers may be nanostructured films madeby: converting a di-p-xylylene paracyclophane dimer to a reactive vaporof monomers, the reactive vapor having a flux; depositing the reactivevapor under vacuum onto a substrate held fixed in a specific angle oforientation relative to the vapor flux to form nanostructuredpoly(p-xylylene) film; reacting the nanostructured poly(p-xylylene) filmwith an agent to form hydrogen atoms attached to the poly(p-xylylene)film that are reactive with a precursor of the chemisorbed group, if thedeposited film does not contain the hydrogen atoms; and reacting thehydrogen atoms with the precursor of the chemisorbed group. The film mayhave nanocolumns, nanohelices, and/or nanochevrons as described in U.S.Patent Application Publication No. 2007/0148206. The nanocolumns may beseparated by channels that are about 10 to about 30 nm wide.

The nature of the fluoroalkylsiloxane films formed during thechemisorption process is known to depend upon the amount of waterpresent on the substrate surface, with multilayer film depositionfavored for chemisorption onto highly hydrated surfaces (Tripp et al.,Langmuir 1993, 9, 3518-3522). Chemisorption can be confirmed by (1) theappearance of a Si 2p signal at 102.7±0.2 eV associated with thepresence of siloxane species (Dressick et al., Jpn. J. Appl. Phys., Part1 1993, 32, 5829-5839) in the XPS spectrum of the treated surface (FIG.5) and (2) the disappearance of the hydroxyl IR band at 3500 cm⁻¹, asshown in spectrum 3 of FIG. 3A. The good wettability (i.e., θ_(w)=78°)measured for the PPX-COHCF₃ films disclosed here suggests awell-hydrated surface, which may be responsible for the formation of afluoroalkylsiloxane multilayer in this system.

The maximum contact angle for aheptadecafluoro-1,1,2,2-tetrahydrodecylsiloxane self-assembled monolayer(SAM) film is reported to be 110° on a planar silicon substrate(Sugimura et al., J. Vac. Sci. Technol. B 2002, 20, 393-395). After thechemisorption of a heptadecafluoro-1,1,2,2-tetrahydrodecylsiloxane filmonto the nanostructured PPX-COHCF₃ film, the contact angle is measuredto be 152° (FIG. 3B). Repetition of theheptadecafluoro-1,1,2,2-tetrahydrodecylsiloxane chemisorption experimentin triplicate for various treatment times (i.e., 1, 3, and 24 h)provides films having θ_(w) values of 104, 152, and 150°, respectively.The heptadecafluoro-1,1,2,2-tetrahydrodecylsiloxane-coatednanostructured PPX-COHCF₃ film clearly becomes superhydrophobic(θ_(w)=152°; i.e., a 42° increase in θ_(w) compared to that of thecorresponding heptadecafluoro-1,1,2,2-tetrahydrodecylsiloxane SAM on aplanar Si surface) after just ˜3 h treatment.

In contrast to the behavior of the nanostructured PPX-COCF₃ film priorto and after LAH treatment, however, water droplets do not adhere(roll-off angle ˜3°) to theheptadecafluoro-1,1,2,2-tetrahydrodecylsiloxane-coated nanostructuredPPX-COHCF₃ film. The penetration of the water droplet into theinterstitial regions between the nanocolumns required to access theintermediate wetting state associated with adhesive behavior is clearlyhindered for this film. Although the low surface energies associatedwith the CF2 and CF3 functional groups present at the outerheptadecafluoro-1,1,2,2-tetrahydrodecylsiloxane layer certainlycontribute to the water repellency, geometric factors may also beinvolved. Specifically, in previous studies it was shown that columnarspacings within nanostructured PPX films can be as small as ˜10-30 nm,especially for thicker films (i.e., micrometer scale) such as those usedhere (Demirel et al., Advanced Materials, 19 (24) 4495-4499 (2007)). Atsuch small dimensions, the chemisorption of a multilayer film of severalnanometers thickness may also contribute to the exclusion of waterthrough narrowing of the channels separating the nanocolumns. The factthat adherent properties have been previously observed for other poorlywetted (i.e., θ_(w)>150°) surfaces indicates that superhydrophobicityalone is not a sufficient barrier to adhesive behavior and thatgeometric or other factors specific to our nanostructured PPX filmscontribute.

The experiments disclosed herein indicate that adhesive behavior canreadily be observed for nanostructured PPX films having higher surfaceenergies (i.e., lower θ_(w)) than those shown in Table 1. For example,the PPX-COHCF₃ film exhibits a relatively high surface energy, asindicated by its low θ_(w)=78°, compared to the materials in Table 1.Unlike the superhydrophobic fluoroalkylsiloxane-coated nanostructuredPPX-COHCF₃ film, however, this film exhibits adhesive behavior similarto that observed for the other nanostructured PPX films described inTable 1 and FIG. 2A. Consequently, adhesive behavior for thesenanostructured PPX films can be observed over an extremely wide range ofsurface energies, presently represented by θ_(w)=78°-136°. It isinteresting that each of these materials possess heteroatom componentscapable of exhibiting dipolar (e.g., PPX-Cl, PPX-Br, or PPX-COCF₃) orhydrogen bonding (e.g., PPX-COHCF₃) interactions with water, which arefeatures that may be necessary to promote the intermediate wetting stateassociated with the adhesive function.

Poly(chloro-p-xylylene) (PPX-Cl) was also selected for thin-filmdeposition. Physical and chemical properties of PPX-Cl films (i.e.,crystallinity, structure, surface energy, and surface topography) werequalitatively assessed by scanning electron microscopy (SEM), atomicforce microscopy (AFM), glancing angle X-ray diffraction (XRD), contactangle measurements, and infrared (IR) spectrophotometry.

Very clearly, the SEM image (FIG. 4A) confirms that nanostructuredPPX-Cl thin films are assemblies of parallel, slanted, free-standingcolumns. Furthermore, the AFM image (FIG. 4B) shows that the columns areassemblies of nanowires that are 50 to 100 nm in diameter. Thus, thenanostructured PPX-Cl thin films contain nanowires having an aspectratio (i.e., length/diameter) as high as 1000:1.

Transmission infrared (IR) spectroscopy was used to compare thenanostructured PPX-Cl thin films qualitatively with planar PPX thinfilms (which are conventionally deposited and do not possessnanostructured morphology). The IR analysis over the 500 to 4000 cm⁻¹frequency range shows features for CH stretching (2800-3000 cm⁻¹),aromatic CH stretching (3026 cm⁻¹), CH deformation (1340 cm⁻¹), Cdeformation (1401 cm⁻¹), and benzene breathing (950 cm⁻¹) in FIG. 4C forboth nanostructured and planar PPX-Cl thin films. This indicates thatthe chemical structure of the nanostructured PPX-Cl thin films is thesame as that of the planar counterparts.

The hydrophobic nature of the nanostructured PPX-Cl thin films is shownby the optical image in FIG. 4D of a water drop on the top surface of afilm. The shape of the water drop on the resulting surface has anapparent contact angle of 100±5°, according to a standard staticcontact-angle measurement system. The surface roughness and porosity ofnanostructured PPX-Cl film were measured with an AFM to be 60 nm and50%, respectively. Hence, the contact angle for the nanostructuredPPX-Cl thin film is predicted by the Wenzel formula to be 94°, which isslightly higher than the 87° value measured for the planar PPX-Cl thinfilm.

The semicrystalline behavior of polymers is of great importance to thephysical and chemical properties exhibited by the material. PlanarPPX-Cl thin films produced conventionally are typically only about50-60% crystalline. At temperatures below the melting point of thecrystallites, the planar PPX-Cl thin films cannot be dissolved, but aslight swelling is observed as the solvent attacks the amorphous phase:0% for water and 1 to 3% for dichlorobenzene. The measured X-raydiffraction (XRD) pattern in FIG. 4E shows that nanostructured PPX-Clthin films are also semi-crystalline. The diffraction pattern of PPX-Clwas also calculated using Mercury software based on a monoclinic unitcell (FIG. 4F) with α=596 pm, b=1269 pm, c (chain axis)=666 pm, andβ=135.2°. A chloro-substituted ring represents the repeat unit along thepolymer chain. Two major peaks, (020) and (−110), are identified fromthe XRD data based on the computed diffraction pattern.

The disclosed films may have the combined properties of a water-dropletcontact angle of greater than about 150 degrees and a water-dropletroll-off angle of about 3 degrees or less, or a water-droplet contactangle of greater than about 70 degrees and a water-droplet roll-offangle of greater than about 10 degrees. The film may also havedirectional hydrophobic or oleophobic properties and directionaladhesive properties. This means that a drop placed on the film mayexhibit different contact angles around the circumference, and will rolloff at different tilt angles depending on the direction of the tilt.This can occur when the nanostructures at the surface of the film areanisotropic as with nanochevrons or tilted nanocolumns. For example, itmay be more difficult for a drop to run off against the grain of tiltedcolumns.

The anisotropic effects may be exploited in such a film on a pivotablesubstrate. The pivot point may be a hinge or other connector thatattaches the substrate to another member, which may be stationary ormovable. The device may be used by tilting the substrate at an angle atwhich the film will adhere a droplet of a predetermined liquid; dippingthe film into a container of the liquid; positioning the substrate overa receptacle; and tilting the substrate at an angle at which the filmdoes not adhere the droplet. Such a device may be used for moving dropsof liquid that comprises one more microbial pathogens or droplets ofradioactive liquids in a shielded environment where people cannot beexposed to the radiation.

An embodiment of such a device is shown in FIG. 6 as a robotic arm.During use, tension is placed on control wire 5 and relaxed on controlwire 6 to rotate the nanostructured parylene film 1 90° clockwise instep 1 to the liquid droplet adhesive position. The robot arm 4 is movedabove a container of liquid 7 and then lowered to contact thenanostructured parylene film 1 with liquid 7 in step 2. The arm 4 isthen raised vertically to remove the nanostructured parylene film 1 fromliquid 7 in step 3, adhering a liquid droplet 8 of liquid 7 to thenanostructured parylene film 1 in step 4. The robot arm 4, withnanostructured parylene film 1 locked in the adhesive position andbearing an adhered droplet 8 of liquid 7 is moved in step 5 above adifferent container 9 designed to receive droplet 8. Tension is placedon control wire 6 and relaxed on control wire 5 to rotate thenanostructured parylene film 1 180° counterclockwise from the liquiddroplet adhesive position to the liquid droplet release position in step6. Droplet 8 of liquid 7 is thereby immediately released from thenanostructured parylene film 1 and falls by gravity into container 9 instep 7. Tension is then placed on control wire 5 and relaxed on controlwire 6 to rotate the nanostructured parylene film 90° clockwise toreturn said nanostructured parylene film 1 in step 8 to its startingposition.

It has been demonstrated that both the hydrophobicity and adhesion ofmodel nanostructured PPX films can be readily varied through control ofthe film morphology (roughness) as well as the surface chemistry via PPXchemical composition or direct chemical modification of the PPX filmsurface. The columnar components of the nanostructured PPX filmsconstitute a carpet of densely packed fibers (i.e., 140-200 nm diameter;˜(2-3)×10⁷ columns mm⁻²) reminiscent of the naturally occurring adhesivekeratinous fibers of a gecko's foot (˜5×10⁵ fibers on each foot). Thenanocolumnar morphology of our PPX films promotes strong adhesiveinteractions with water droplets over a wide range of surfacecompositions, chemistries, and energies, as measured by θ_(w) valuesranging from 78 to 136°. Further attempts to reduce wettability viachemisorption of fluoroalkylsiloxane coatings onto nanostructured PPXfilms produce “truly” superhydrophobic materials exhibiting both poorwettability (i.e., θ_(w)=152°) and poor water droplet adhesion (waterdrop roll-off angle <3°) under these conditions. Nevertheless, theability to control both hydrophobicity and adhesion using nanostructuredPPX films is a promising development because it may lead to a newgeneration of coatings for applications ranging from self-cleaningsurfaces and biomedical implants to robotics.

The following examples are given to illustrate specific applications.These specific examples are not intended to limit the scope of thedisclosure in this application.

EXAMPLE 1

Characterization Methods—Transmission infrared spectra (IR, Bruker) werecollected with respect to a silicon wafer reference in air. Spectra wererecorded using Norton-Beer apodization with 1 cm⁻¹ resolution, and foreach spectrum, 400 scans were coadded. The spectra were analyzed usingOPUS 5.5 software. The surface roughness was quantified by AFM(Nanoscope E, Veeco) in a liquid chamber using silicon nitridecantilevers in contact mode. Column dimensions were calculated withNanoscope software (Veeco Metrology, CA) using the grain size tool.Contact angle measurements (DFTA 1000, First Ten Angstroms, Inc.) werecarried out with a video microscope interfaced to a computer. Theglancing angle XRD pattern of a nanostructured PPXC thin film depositedon a glass substrate was measured (Scintag Pad V). Samples for scanningelectron microscopy (JEOL 6700F FE-SEM) images were prepared by cleavingthe thin films in liquid nitrogen.

The film roughness of the nanostructured PPX films can be tailored byvarying the thickness of film. Surface area and roughness of each samplewere calculated with the Nanoscope Software (Veeco Metrology, CA). Ingeneral, the nanostructured PPX films were 30% rougher compared to theplanar PPX films. This is expected since planar films form a conformalsurface and their surface roughness is very small. The contact anglemeasurement is performed using a standard water drop test. FIG. 4D showsthat the film become more hydrophobic as the surface roughness ofnanostructured PPX increases.

Contact angles for the films ranged from 80° to 135° depending on thetype of film. In general, every planar film was less hydrophobic thanits nanostructured counterparts, because the contact angle for theformer is less than for the latter. Nanostructured PPX-F exhibitedsuperior hydrophobicity compared to the remaining five types of films.All surface measurements were performed with a Nanoscope-E atomic forcemicroscope (Veeco Metrology, CA). Topography images were collected inambient air at room temperature, with silicon nitride (SiN) triangularcantilevers having contact mode tips (DNT-20, Veeco Metrology, CA).Static contact angles were measured by applying a video microscopeinterfaced to a computer (FTA-I 000).

EXAMPLE 2

PPX Film Deposition—The paracyclophane precursor for thepoly(chloro-p-xylylene) (PPX-Cl) films, dichloro-di-p-xylylene, wasobtained from Uniglobe-Kisco Inc. The precursors for thepoly-(bromo-p-xylylene) (PPX-Br) film, dibromo-di-p-xylylene, andpoly(trifluoroacetyl-p-xylylene) (PPX-COCF₃) film,trifluoroacetyl-di-p-xylylene, were prepared according to the literaturemethod (Cetinkaya et al., Polymer 2007, 48, 4130-4134). The nanocolumnarPPX films used here were prepared using an α=10° deposition angle.

EXAMPLE 3

Film Characterization—Static contact angles were measured by applying avideo microscope interfaced to a computer (FTA-1000) to capture dropimages. Deionized H₂O (10 μL, 18 MΩ-cm resistivity; Barnstead NanopureII deionizer) was used for contact angle measurements. Surface roughnesswas measured with a Nanoscope-E atomic force microscope (VeecoMetrology, CA) in ambient air at room temperature, with silicon nitride(SiN) triangular cantilevers having contact mode tips (DNP-20, VeecoMetrology, CA). The rms roughness from 5 μm scans (average of threesets) was calculated using the Nanoscope software without tipdeconvolution methods as a result of high roughness. Scanning electronmicroscope (SEM) images were recorded with a Philips XL-40 system aftercleaving the samples in liquid nitrogen. The FTIR (Thermo Nicolet IR)and ATR (diamond crystal) data were collected with respect to siliconwafer reference in air and were recorded using Norton-Beer apodizationwith 4 cm⁻¹ resolution. For each spectrum, 100 scans were co-added.

EXAMPLE 4

PPX-COCF₃ Film Reduction by LiAH₄—Experiments were done in a glove bagin a dry N₂ atmosphere. All chemicals were used without furtherpurification. LiAlH₄ (LAH) was purchased from Alfa Aesar. In a typicalexperiment, 50 mg of LAH was dissolved in ˜25 mL of THF (AldrichSure-Seal, <0.005% H₂O) in a dry 50 mL round-bottomed flask, and thePPX-COCF₃ film was immersed in this solution for 4 h. The treatedPPX-COCF₃ film, designated PPX-COHCF_(3,) was removed from the flask,rinsed with fresh THF, removed from the glove bag, and examined by IR.Upon completion of the reaction, the IR absorption band of the ketonegroup at 1712 cm⁻¹ disappeared. The reduced PPX-COHCF₃ film was thenthoroughly rinsed with water (doubly distilled) and dried under vacuumovernight.

EXAMPLE 5

SilaneFunctionalization—Heptadecafluoro-1,1,2,2-tetrahydrodecyltrichlorosilane(Gelest, PA) was reacted with the PPX-COHCF₃ film in the glove bag underdry N₂. A 1% (w/v) solution (10 mL) of the organotrichlorosilane wasprepared in hexane. The PPX-COHCF₃ film was immersed in this solutionand kept there for 1, 3, or 24 h. Upon completion of the reaction, thefilm was removed from the organotrichlorosilane solution, washed withhexane, and dried under vacuum overnight.

Obviously, many modifications and variations are possible in light ofthe above teachings. It is therefore to be understood that the claimedsubject matter may be practiced otherwise than as specificallydescribed. Any reference to claim elements in the singular, e.g., usingthe articles “a,” “an,” “the,” or “said” is not construed as limitingthe element to the singular.

What is claimed is:
 1. A polymer comprising:

wherein n is a positive integer.
 2. The polymer of claim 1, wherein thepolymer is a nanostructured film made by: converting atrifluoroacetyl-di-p-xylylene paracyclophane dimer to a reactive vaporof monomers, the reactive vapor having a flux; depositing the reactivevapor under vacuum onto a substrate held fixed in a specific angle oforientation relative to the vapor flux to form nanostructuredpoly(trifluoroacetyl-p-xylylene) film; and reacting the nanostructuredpoly(trifluoroacetyl-p-xylylene) film with a suitable reducing agent toform the polymer.